The present invention relates to pellicles that are used in the semi-conductor chip industry, and more particularly to a wet die cutter assembly and method usable in connection with the manufacture of pellicles.
In the semi-conductor chip industry it is well known that pattern transfer from the photo mask to substrate is accomplished by directing light from a suitable light source through the mask to the substrate. During the pattern transfer process, also called the photolithographic process, the patterns on the photo mask are projected onto the substrate which has been treated with a photo-sensitive substance. This results in the mask pattern being reproduced on to the substrate.
A manufacturing problem is encountered when foreign substances are present on the surface of the mask. There is a problem because foreign substances on the mask surface will also be reproduced on the substrate and therefore will interfere with proper pattern transfer.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on it. The pellicle membrane extends parallel to the mask, and at a predetermined distance spaced away from the mask. Any contamination which would ordinarily land on the mask surface, falls onto the pellicle membrane rather than onto the mask.
Pellicles eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The use of pellicles can increase the quality of the resulting circuit, thereby dramatically improving circuit fabrication productivity. Consequently, it is no surprise that pellicle manufacturing techniques have become increasingly important because high quality pellicles are critical to the success of the photolithographic process.
During the pellicle manufacturing process, it is important to minimize the possibility of either relatively large or small contaminant particles being deposited on the pellicle membrane. Relatively large particles are unacceptable because they may be reproduced in the substrate during photolithography even though they are out of focus. Equally unacceptable are particles (whether large or small) that are deposited on the underside of the pellicle membrane or the pellicle frame. Such particles may drop onto the mask surface during photolithography which is precisely what is to be avoided by using pellicles.
It is also critical that the pellicle membrane be extremely uniform across its surface. Membrane uniformity ensures that light passing through the membrane during photolithography will not be obstructed or refracted in any way. The composition of the membrane must be highly uniform, and the membrane must be tensioned evenly across the pellicle frame. Also, it is important to ensure that a continuous seal exists between the relatively thin membrane and the frame.
To further understand these important requirements, it is necessary to provide an explanation of how pellicles are formed.
As is known in the art, forming an optical membrane is the first step in pellicle manufacture. Commonly the membrane is prepared by spinning a suitable polymer, such as nitrocellulose or a nitrocellulose-containing polymer, on a substrate. The newly formed membrane is then removed from the substrate and held under tension, adjacent its periphery, to prepare it for subsequent manufacturing steps.
Next, a frame is fastened or bonded to a working area of the membrane, framing the working area. After fastening, the framed working area is ready to be separated from the remaining area of the membrane.
It is this separating step that is of the utmost importance to high quality pellicle manufacture. For it is during the separating step that the framed working area must be cut away from the remainder of the membrane. Using the assembly and method of the present invention, a surprisingly successful separation is obtained.
Currently, it is known to remove the framed working area by cutting the membrane outward of the frame using a suitable knife, or razor blade. Alternatively, solvent or heat may be used in a conventional process to cut the membrane. The problem with such a procedure is that the initial puncturing of the membrane by the knife causes "shattering" of the membrane in the non-working area which produces contaminant particles that may collect on the frame or working area.
Additionally, the membrane may tear in undesired directions. If the tear proceeds through the bond between the frame and the membrane and into the working area, the pellicle must be rejected because it will not perform its function when the integrity of the working area is damaged.
An even greater problem exists when the tear passes into but not through the bond between the frame and the membrane. Such a tear will vary the tension of the membrane across the working area which will cause microscopic distortion that can affect the uniformity of the membrane. This distortion, which may not be visible to the naked eye, can be catastrophic because it is essential that the pellicle membrane transmit light uniformly without refraction. While equipment is often used to detect membrane distortion, such equipment is expensive and its use can slow down the production process.
Accordingly, it is a general object of the present invention to provide a unique assembly and method for cutting a membrane.
Another object of the invention is to provide a unique assembly and method for promoting shatter-free, tear-free separation of a framed working area from the remainder of a membrane.